Hey John,
Congrats on this exciting and new time for you. Wehn I did supply preaching I always used texts that I had recently studied in seminary, very much enjoyed learning about, and would ask the professors for resource advice every week. That way you're already ahead on the exegesis and can concentrate on organizing and developing the sermon further. I think it's important to feel confident in your understanding of the text (important for you and the congregants). Also, don't feel like you have to do a series yet. However, I John is an easily accessible text for seminary students (b/c in a series you'll have to develop each text in the context of the whole). The Beatitudes are also good. A thematic series might also be something you'll enjoy doing. God Bless.
